Sodium-ion batteries have exciting energy storage potential for a large number of applications. Cost is an important factor in transportation batteries and other large format applications because of their sheer size (amount of packed energy) and the number of electrodes that must be integrated into the battery pack. A battery that uses Na ions instead of Li ions is attractive, for example, due to potential cost advantages for sodium relative to lithium. Sodium carbonate (also known as “soda ash” and the mineral “trona”) is a major potential resource for Na-ion battery materials. The United Stated Geological Survey has estimated that, as of 2011, reserves of sodium carbonate in the U.S. are in the range of about 23 billion tons, at an as-mined cost of only about $135 per ton. Battery pack engineering modeling calculations estimate that a Na-ion battery operating at 3.3 V, employing an anode (negative electrode) with a capacity of 500 mAhg−1, and a cathode with a capacity of 200 mAhg−1, will provide upwards of 15% to 20% higher energy density (210 Whkg−1, gravimetric basis) compared to current Li-ion batteries.
Na-ion cathode and anode materials have seen some recent major advances. There are only a few studies of carbon and alloy anodes in the current scientific literature. The list of cathode materials is much larger, however, including layered oxides, poly-anionic materials, fluorides, framework oxides, NASICONs, and sulfate-fluorides. While there are many examples of cathodes in the literature, there are limitations to the structures that can be used. The Na ion is about 55% larger in radius than Li ion, and prefers only octahedral coordination (6-coordinate bonding), as opposed to the preferred tetrahedral coordination (4 coordinate bonding) of lithium. Consequently, it is difficult to find a suitable host material to accommodate Na ions and allow reversible and rapid ion insertion and extraction.
Various transition metal sulfides can be employed in Na batteries, as well. The early first-row transition metal layered sulfides possess suitable electrochemical properties, and the first work was directed primarily to binary metal sulfides, most notably, the prototype intercalation electrode, TiS2. The Ti(+3)/(+4) redox couple is about 1.8 to 2.0 V versus Na. Layered TiS2 provides soft bonding of Na to the sulfide atoms in the layers, and large gallery space between transition metal (TM) layer slabs to fit Na+, which allows facile reversible electrochemical intercalation and deintercalation (also referred to herein, for convenience, as “(de)intercalation”). The electrochemical properties of other disulfides such as VS2, TaS2, ZrS2, NbS2Cl2 have been elaborated by Abraham et al., (Solid State Ionics, 1982:7, 199) in molten salt cells, but because of the molten-salt nature of the electrolytes, these electrodes were operated at 130° C.
There is an ongoing need for sodium cells that can be operated at lower temperatures, such as ambient room temperature (e.g., 20 to 25° C.). The electrochemical cells described herein address this need.